1. Field of the Invention
The present invention relates to a technique for improving the crystal quality of a group-III nitride crystal and, more particularly, to a technique for improving the flatness of a crystal surface.
2. Description of the Background Art
A group-III nitride crystal has been used as a material constituting a semiconductor device such as a photonic device and an electronic device, and has gained the spotlight in recent years as a semiconductor material constituting a rapid IC chip for use in a portable telephone. In particular, an AlN film has received attention as a material for application to a field emitter.
For such device applications, a group-III nitride crystal is ideally provided as free-standing one. Under the current circumstances, however, a group-III nitride crystal is typically provided in the form of a so-called epitaxial substrate such that the group-III nitride crystal having a thickness of the order, at most, of 10 μm (to such a degree that no warpage resulting from a difference in coefficient of thermal expansion occurs) is epitaxially formed on a predetermined single crystal base because of problems with crystal quality, manufacturing costs and the like. In general, thin film formation methods such as an MOCVD (metal-organic chemical vapor deposition) process and an MBE (molecular beam epitaxy) process are used to form such an epitaxial substrate.
In the epitaxial substrate having such a construction, however, a difference in lattice constant exists between the base and the group-III nitride crystal to give rise to dislocations resulting from such a lattice mismatch at an interface therebetween. Such dislocations thread through the group-III nitride film serving as a device functional layer, and most of the dislocations propagate to the surface thereof. To attain good device characteristics, there is a need to minimize the dislocations propagating to the device functional layer.
An ELO process has been proposed to improve the crystal quality resulting from the lattice mismatch between the base and the group-III nitride crystal. See, for example, Akira Sakai and Akira Usui, “Reduction of dislocation density in GaN films by epitaxial lateral overgrowth,” “OYO BUTURI,” The Japan Society of Applied Physics, Vol. 68, No. 7, pp. 774-779 (1999). The process utilizes an epitaxial lateral overgrowth technique using a mask and the like to reduce a dislocation density.
Selective area growth can not be expected when the above-mentioned group-III nitride crystal is composed of AlN. To solve the problem, a technique for reducing the number of dislocations using the ELO process using the base with a shaped surface (as disclosed, for example, in Japanese Patent No. 3455512) and a method of forming AlN with a small number of dislocations by directly nitriding a sapphire substrate (as disclosed, for example, in Japanese Patent Application Laid-Open No. 2004-137142) have been proposed.
It is necessary to improve the crystal quality as much as possible in order to increase the performance of a functional device when a group-III nitride crystal is formed on a base for the function as the functional device. For instance, when an epitaxial substrate is used for a functional device, it is typical that a single crystal layer is further formed on the epitaxial substrate. The good crystal quality of the single crystal layer requires the good flatness of the surface of the group-III nitride crystal constituting the epitaxial substrate. The good flatness of the surface as used herein shall mean, for example, the absence of pits, low surface roughness (preferably, a flat surface at the atomic level), and the like.
It is also necessary to minimize the dislocation density. The reduction in dislocation density promises to achieve, for example, an increase in light emitting efficiency for a light-emitting device, a decrease in dark current for a photodetector device, and an increase in mobility for an electronic device.
The above-mentioned background art techniques can achieve the reduction in the number of dislocations, but are not intended to improve the surface flatness.